Process for friction stir welding dissimilar metals and workpiece assemblies formed thereby

ABSTRACT

In a process for friction stir welding together pieces of dissimilar material, a first piece of a second metal is overlaid onto a first piece of a first metal that is dissimilar from the second metal such that at least a portion of the first piece of second metal overlaps a portion of the first piece of first metal. The first piece of second metal has a plurality of holes therein and the holes are disposed in overlapping relationship with the portion of the first piece of first metal. Each of the holes is filled with a plug formed from the first metal. The first piece of first metal is friction stir welded to the first piece of second metal at each of the plug locations.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

This invention was made with government support under Grant Award No.0605622 awarded by the National Science Foundation (NSF). The governmentmay have certain rights in the invention.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent applicationSer. No. 12/589,948, filed Oct. 30, 2009, now U.S. Pat. No. 8,464,926,issued Jun. 18, 2013, which is incorporated herein by reference.

BACKGROUND

The field of the invention relates generally to friction stir weldingand more particularly to friction stir welding of dissimilar metals andto workpiece assemblies formed by friction stir welding dissimilarmetals together.

Recent surveys conducted by the Joining and Welding Research Institute(JWRT) of Japan and the Edison Welding Institute (EWI) of the U.S. haveidentified welding of dissimilar metals as a top priority in materialsjoining technologies. For instance, being able to weldaluminum-to-copper would be advantageous in many industries whereelectric connections are made during the manufacturing process. Inanother instance, being able to weld aluminum-to-steel oraluminum-to-magnesium would result in significant weight reduction insome applications, which would be advantageous in many industries, e.g.,the aircraft, locomotive, shipbuilding, and automotive manufacturingindustries. Being able to efficiently and effectively weldaluminum-to-magnesium is of particular interest because magnesium has aspecific strength (i.e., strength to density ratio) that is 14 percenthigher than aluminum and 68 percent higher than iron making it one ofthe lightest metallic structural materials. Although aluminum andmagnesium alloys are typically soft materials and often have relativelysimilar melting points, they tend to react with each other when heatedsuch as during friction stir welding. The more they are heated up, thatis, the more heat input during friction stir welding, the more theyreact with each other to weaken the resultant weld.

As illustrated in FIGS. 1 and 2, friction stir welding is typicallypreformed using a cylindrical tool T having a pin P extending downwardtherefrom. During use, the pin P is rotated at a constant speed and fedat a constant traverse rate along a joint line JL between two pieces ofsheet or plate material P1, P2. The pieces P1, P2 can be butt welded(side-to-side) together, as illustrated in FIG. 1, or lap welded(overlapped) together, as illustrated in FIG. 2. The pin P of the tool Ttypically has a length that is slightly less than the desired welddepth. During the welding process, a shoulder S of the tool T is oftenin direct contact with an upper surface of both the pieces P1, P2 beingwelded. As a result, heat is generated by the friction between the toolshoulder S and pin P and the pieces P1, P2 being welded. This heatcauses the pieces P1, P2 to soften about the joint line JL withoutreaching their melting point. In other words, the friction stir weldingprocess causes both of the pieces P1, P2 to plasticizes adjacent thejoint line JL. As the tool T is fed transversely with respect to thepieces P1, P2, a leading face of the pin P directs the plasticizedmaterial toward a back of the pin. Angling the pin P by 2-4 degrees inthe transverse direction facilitates feeding the pin through the piecesP1, P2 and directing the plasticized material toward the back of thepin.

With reference to FIG. 1, prior efforts have been made to butt weld analuminum alloy to a magnesium alloy using friction stir welding. Inconventional butt welding using friction stir welding, an end of a pieceof aluminum alloy (e.g., piece P1) is accurately aligned with and placedagainst an end of a piece of magnesium alloy (e.g., piece P2) to form aseam or joint line TL. The tool T and thereby the pin P are fed alongthe joint line JL (i.e., in a longitudinal direction with respect to thetwo pieces P1, P2 as viewed in FIG. 1). The pin P can be coaxial withthe joint line JL or offset toward either the piece of aluminum alloy P1or the piece of magnesium alloy P2. In addition, the piece of aluminumalloy P1 can be placed on either the advancing side, as illustrated inFIG. 1, or retreating side of the tool T. The piece of magnesium alloyP2 is placed on the opposite side from the piece of aluminum alloy P1.In the illustrated configuration, the piece of magnesium alloy P2 isplaced on the retreating side of the tool T. While the butt weld formedbetween the pieces of aluminum and magnesium alloys P1, P2 can berelatively strong, strict tolerances and controls are needed during thewelding process to form such a weld thereby making it difficult,relatively time consuming, and costly.

As illustrated in FIG. 2, prior efforts have also been made to lap welda piece of aluminum alloy (e.g., piece P1) to a piece of magnesium alloy(e.g., piece P2). Lap welding is typically preferred by manufacturersbecause the tolerances and controls needed during the welding processare substantially less than those needed during the butt weldingprocess. As seen in FIG. 2, lap welding is preformed by overlapping aportion of one of the pieces with a portion of the other piece. In theillustrated configuration, the piece of magnesium alloy P2 overlaps thepiece of aluminum alloy P1. The overlapped portions of the pieces P1, P2are welded together using friction stir welding. That is, the overlyingpiece P2 is friction stir welded to the underlying piece P1.

The pieces P1, P2 can be welded together along a single joint line usingconventional single-pass lap welding or can be welded together along twojoint lines using conventional double-pass lap welding. Whenconventional double-pass lap welding is used, the pieces P1, P2 ofmaterials are flipped over after they have been welded on one side andare then welded on the opposite side using the same process. Whendissimilar metals (e.g., aluminum alloy and magnesium alloy) are lapwelded together using friction stir welding, brittle intermetalliccompounds (e.g., Al₁₂Mg₁₇, Al₃Mg₂) are often formed, which severelydegrades the strength of the weld. As a result, prior efforts to lappieces of dissimilar materials have been relatively unsuccessful.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, a process for friction stir welding pieces of dissimilarmetals together generally comprises overlying a first piece of a secondmetal onto a first piece of a first metal that is dissimilar from thesecond metal such that at least a portion of the first piece of secondmetal overlaps a portion of the first piece of first metal. The firstpiece of second metal has a plurality of holes therein and the holes aredisposed in overlapping relationship with the portion of the first pieceof first metal. Each of the holes are filled with a plug formed from thefirst metal. The first piece of first metal is friction stir welded tothe first piece of second metal at each of the plug locations.

In another aspect, a workpiece assembly generally comprises a firstpiece of a first metal and a first piece of a second metal overlying atleast a portion of the first piece of first metal. The first piece ofsecond metal has a plurality of holes therein and each of the holes isfilled with a plug formed from the first metal. A discontinuous weldseam securely joins the first piece of first metal and the first pieceof second metal. The weld seam is defined by a plurality of weld spotscomprising a stirred mixture of the first metal and the second metal.Each of the weld spots correspond to one of the plugs filling the holesin the first piece of second metal.

BRIEF DESCRIPTION OF THE DRAWINGS

This patent or patent application publication contains at least onedrawing executed in color. Copies of this patent or patent applicationpublication with color drawing(s) will be provided by the Office uponrequest and payment of the necessary fee.

FIG. 1 is a perspective illustrating a conventional butt welding processusing friction stir welding.

FIG. 2 is a perspective illustrating a conventional single-pass lapwelding process using friction stir welding.

FIG. 3 is a perspective illustrating a modified single-pass lap weldingprocess using friction stir welding.

FIG. 4 is a cross-section taken along line 4-4 of FIG. 3.

FIG. 5 is a cross-section similar to that of FIG. 4 but illustrating aworkpiece assembly formed by the pieces being welded together using theprocess illustrated in FIGS. 3 and 4.

FIGS. 6A and 6B are perspectives illustrating a modified double-pass lapwelding process using friction stir welding.

FIG. 7 is a perspective illustrating a workpiece assembly formed usingthe double-pass lap welding process illustrated in FIGS. 6A and 6B.

FIG. 8 is a cross-section taken along line 8-8 of FIG. 7.

FIGS. 9A-9D are perspectives illustrating a modified lap welding processusing spot friction stir welding.

FIG. 10 is a perspective illustrating a workpiece assembly formed usingthe lap welding process illustrated in FIGS. 9A-9D.

FIG. 11 is a cross-section taken along line 11-11 of FIG. 10.

FIG. 12 is a binary Al—Mg phase diagram.

FIG. 13 is a box diagram related to experiments conducted using varioustechniques of friction stir welding.

FIG. 14 is a diagram illustrating the effect of material positions onthe joint strength in butt friction stir welding of an aluminum alloyand a magnesium alloy.

FIGS. 15A and 15B are graphical comparisons of thermal cycles measuredin weld B-7 and B-11 of FIG. 14.

FIGS. 16A and 16B are color photographs of transverse cross-sections ofwelds B-7 and weld B-11, respectively.

FIG. 17 provides a diagram and two color photographs of a single-passconventional lap weld of an aluminum alloy and a magnesium alloy whereinthe magnesium alloy is on top (CL-2 of FIG. 18).

FIG. 18 is a diagram illustrating the effect of material positions onthe joint strength in lap friction stir welding of an aluminum alloy anda magnesium alloy.

FIGS. 19A and 19B are color photographs of transverse cross-sections ofwelds CL-1 and CL-2 of FIG. 18, respectively.

FIG. 20A illustrates a conventional lap friction stir welding process ofdissimilar metals.

FIG. 20B illustrates a modified lap friction stir welding process ofdissimilar metals.

FIG. 21 is a diagram and two color photographs of a single-pass modifiedlap weld of an aluminum alloy and a magnesium alloy wherein themagnesium alloy and a small piece of the aluminum alloy are on top (ML-2of FIG. 22).

FIG. 22 is a diagram illustrating the effect of material positions onthe joint strength in single-pass modified lap friction stir welding ofan aluminum alloy and a magnesium alloy.

FIG. 23 is a graphical comparison of the tensile test curves of the bestsingle-pass conventional lap weld CL-2 (FIG. 18) and the bestsingle-pass modified lap weld ML-1 (FIG. 22).

FIGS. 24A and 24B are color photographs of transverse cross-sections ofwelds ML-3 and ML-1 of FIG. 22, respectively.

FIG. 25 is a diagram illustrating the effect of material positions onthe strength of dual-pass lap welds made between an aluminum alloy and amagnesium alloy.

FIG. 26 graphically illustrates a comparison of the tensile test curvesof welds CL-7 and ML-6 (FIG. 25).

FIG. 27 graphically illustrates a comparison of the tensile teststrength and plug diameter using friction stir spot welding.

DETAILED DESCRIPTION OF THE DRAWINGS

With reference now to the drawings, FIGS. 3-5 illustrated one embodimentof a process of friction stir welding workpieces of dissimilar metalstogether and the workpiece assembly resulting therefrom. In thisembodiment, the pieces of dissimilar metals are welded together using aprocess referred herein as “modified single-pass lap welding”, which isdescribed below. It is understood that the pieces of dissimilar metalscan be any suitable, dissimilar metals or alloys, e.g., aluminum andmagnesium; aluminum and copper; aluminum and steel; aluminum andtitanium, that are capable of being friction stir welded together. It isalso understood that the pieces of dissimilar metals can be formed fromdifferent alloys of the same metal, e.g., 2000-series Al alloys can bewelded to 6000-series Al alloys, 6000-series Al alloys can be welded toAl casting alloys.

As seen in the embodiment illustrated in FIG. 3, a first piece of afirst metal (e.g., a first piece of magnesium alloy 10) is laidgenerally flat on a suitable support surface 12. A first piece of asecond, dissimilar metal (e.g., a first piece of aluminum alloy 20) isoverlaid on first piece of magnesium alloy 10. A second piece of thefirst metal (e.g., a second piece of magnesium alloy 30) is alsooverlaid on the first piece of magnesium alloy 10 and in side-to-siderelationship with the first piece of aluminum alloy 20. As a result ofthe relative arrangement between the second piece of the magnesium alloy30 and the first piece of aluminum alloy 20, a joint line 40 is formedbetween the two pieces. Each of the illustrated pieces 10, 20, 30 is asheet of metal having substantially the same thickness as the otherpieces. It is understood, however, that the pieces 10, 20, 30 can beformed from plates of metal, which are thicker than the illustratedsheets, or any other suitable metal. It is also understood that thepieces 10, 20, 30 can have different thicknesses as compared to eachother. That is, one or two of the pieces may have a thickness that isdifferent from the other pieces.

As seen in FIG. 3, the first pieces of magnesium alloy 10 and aluminumalloy 20 are generally rectangular in shape but can have differentsuitable shapes without departing from the scope of this invention. Inthe illustrated configuration, the first pieces of magnesium alloy 10and aluminum alloy 20 have substantially the same shape but it isunderstood that the pieces can have different shapes. That is, the firstpiece of magnesium alloy 10 can have a shape that is different from thefirst piece of aluminum alloy 20. The illustrated second piece ofmagnesium alloy 30 is also rectangular but has a width that issubstantially less than the widths of the first piece of magnesium alloy10 and the first piece of aluminum alloy 20. The length of the secondpiece of magnesium alloy 30 is substantially the same as the lengths ofthe first piece of magnesium alloy 10 and the first piece of aluminumalloy 20. It is contemplated that the length and width of the secondpiece of magnesium alloy 30 can be different than that illustrated inFIG. 3. For example, the second piece of magnesium alloy 30 can have alength that is less than the lengths of the first piece of magnesiumalloy 10 and/or the first piece of aluminum alloy 20.

As seen in FIG. 3, the three pieces 10, 20, 30 can be friction stirwelded together along the joint line 40. In one suitable embodiment, aconventional milling machine (not shown) can be equipped with a tool 50having a pin 52 extending downward therefrom. The tool 50 and pin 52 canbe rotated either counterclockwise, as indicated by arrow 54 in FIG. 3,or clockwise and moved transversely along the joint line 40, asindicated by arrow 56 in FIG. 3. In the illustrated embodiment, the tool50 and pin 52 are aligned along the joint line 40 but it is understoodthat the tool and pin can be offset slightly from the joint line. Thetool 50 and pin 52 can be offset toward either the second piece ofmagnesium alloy 30 or the first piece of aluminum alloy 20. Suitably,the amount of offset is equal to or less than the diameter of the pin52.

In one suitable embodiment, the pin 52 is tilted forward in thedirection of movement of the tool 50 between about 2 degrees and about 4degrees. More suitably, the pin 52 is tilted forward about 3 degrees. Aplurality of clamps 60 can be used to secure the pieces 10, 20, 30during the welding process. It is contemplated that other means forsecuring the pieces 10, 20, 30 during the welding process can be usedbesides the illustrated clamps 60 and that more or fewer clamps can beused.

During the welding process, the tool 50 and thereby the pin 52 is movedalong the joint line 40 to weld the three pieces 10, 20, 30 together toform a workpiece assembly, indicated generally at 62 in FIG. 5. A lengthof the pin 52 is predetermined such that it extends through the secondpiece of magnesium alloy 30 and first piece of aluminum alloy 20, andinto but not completely through the first piece of magnesium alloy 10 asillustrated in FIG. 4.

As discussed above, the tool 50 and the pin 52 generate a sufficientamount of heat during the welding process to plasticize portions of thepieces 10, 20, 30 adjacent the joint line 40 as they are moved along thejoint line. The pin 52 mixes the plasticized material together to form aweld seam 64 (FIG. 5). In the embodiment illustrated in FIGS. 3 and 4,the plasticized material is mixed (or stirred) by the pin 52 in acounterclockwise direction. As a result, plasticized material from thefirst and second pieces of magnesium alloy 10, 30, which is on what isreferred to as the advancing side, is moved toward and mixed withplasticized material of the first piece of the aluminum alloy 20, whichis on what is referred to as the retreating side. Plasticized materialfrom the first piece of aluminum alloy 20 is also moved toward and mixedwith plasticized material from the first and second pieces of magnesiumalloy 10, 30. The plasticized materials of the pieces 10, 20, 30 areallowed to cool and form the weld seam 64, which securely joins thethree pieces together to define the workpiece assembly 62.

With reference now to FIG. 5, the above-described process results in theworkpiece assembly 62 comprising the first and second pieces ofmagnesium alloy 10, 30 and the first piece of aluminum alloy 20 beingwelded together by weld seam 64. The weld seam 64 is defined by astirred mixture of magnesium and aluminum alloys.

With reference now to FIGS. 6A-8, another suitable process of frictionstir welding pieces of dissimilar metals together and the resultingworkpiece assembly are illustrated therein. In this embodiment, thepieces of dissimilar metals are welded together using a process referredto herein as “modified dual-pass lap welding”, which is described below.In the illustrated embodiment, a first piece of a first metal (e.g., afirst piece of magnesium alloy 110) is laid generally flat on a suitablesupport surface 112. A first piece of a second, dissimilar metal (e.g.,a first piece of aluminum alloy 120) is overlaid on the first piece ofmagnesium alloy 110. A second piece of the first metal (e.g., a secondpiece of magnesium alloy 130) is also overlaid on the first piece ofmagnesium alloy 110 and in side-to-side relationship with the firstpiece of aluminum alloy 120. As a result of the relative arrangementbetween the second piece of magnesium alloy 130 and the first piece ofaluminum alloy 120, a joint line 140 is formed between the two pieces.

As seen in FIG. 6A, the three pieces 110, 120, 130 can be friction stirwelded together along the joint line 140 using the tool 50 and the pin52 extending downward from the tool in the same manner describe abovewith respect to FIGS. 3-5. That is, the tool 50 and pin 52 can berotated either counterclockwise, as indicated by arrow 54 in FIG. 6A, orclockwise and moved transversely along the joint line 140, as indicatedby arrow 56 in FIG. 6A. In the illustrated embodiment, the tool 50 andpin 52 are aligned for movement along the joint line 140 but it isunderstood that the tool and pin can be offset slightly from the jointline. The tool 50 and pin 52 can be offset toward either the secondpiece of magnesium alloy 130 or the first piece of aluminum alloy 120.

In one suitable embodiment, the pin 52 is tilted forward in thedirection of movement of the tool between about 2 degrees and about 4degrees, and more suitable, the pin 52 is tilted forward about 3degrees. As in the previous process, the plurality of clamps 60 can beused to secure the pieces 110, 120, 130 during the welding process.

During the welding process, the tool 50 and thereby the pin 52 is movedalong the joint line 140 to weld the three pieces 110, 120, 130together. After the three pieces 110, 120, 130 are welded together alongjoint line 140, the flash is removed. Then, the pieces are flipped oversuch that the first piece of magnesium alloy 110 overlies the firstpiece of aluminum alloy 120 (FIG. 6B). A second piece aluminum alloy 132is overlaid on the first piece of the aluminum alloy 120 and inside-to-side relationship with the first piece of magnesium alloy 110.As a result of the relative arrangement between the second piece of thealuminum alloy 120 and the first piece of magnesium alloy 110, a jointline 142 (broadly, “a second joint line”) is formed between the twopieces. As seen in FIG. 6B, the three pieces 110, 120, 132 can befriction stir welded together along the joint line 142 using the tool 50and pin 52. The tool 50 and the pin 52 are moved along the joint line142 or slightly offset from the joint line to weld the three pieces 110,120, 132 together.

With reference now to FIGS. 7 and 8, the above-described process resultsin the workpiece assembly 162 comprising the first and second pieces ofmagnesium alloy 110, 130 and the first and second pieces of aluminumalloy 120, 132 being welded together by weld seams 164, 164′. The weldseams 164, 164′ are defined by a stirred mixture of magnesium andaluminum alloys.

FIGS. 9A-11 illustrate yet another process of friction stir weldingpieces of dissimilar metals together to form a workpiece assembly 262.In this embodiment, the pieces of dissimilar metals are welded togetherusing a process referred to herein as “modified spot friction stirwelding”, which is described below. In the illustrated embodiment, afirst piece of a first metal (e.g., a first piece of magnesium alloy210) is laid generally flat on a suitable support surface 212. A firstpiece of a second, dissimilar metal (e.g., a first piece of aluminumalloy 220) is overlaid on the first piece of magnesium alloy 210.

As illustrated in FIG. 9A, the tool 52 is used to drill a plurality ofholes 225 into the first piece of aluminum alloy 220. In the illustratedembodiment, the holes 225 are generally circular and are spaced fromeach other along a transverse joint line 240 (FIG. 9B). It iscontemplated that all or some of the holes can have different sizes, beother than circular, and arranged in any suitable configuration. It isalso contemplated that the holes 225 can be drilled using a differenttool or formed by a different means than drilling (e.g., stamping). Itis further contemplated that the holes 225 may extend into but notthrough the first piece of magnesium alloy 220 thereby creating recessesin the first piece of magnesium alloy that correspond to each of theholes. Moreover, the holes 225 may be formed in the first piece ofaluminum alloy 220 before it is overlaid on the first piece of magnesiumalloy 210.

With reference now to FIG. 9C, a plug 230 of magnesium alloy is placedin each of the holes 225. Each of the illustrated plugs 230 are sizedand shaped to substantially fill one of the holes 225. In one suitableembodiment, the plugs 230 have a height that is substantially the sameas the depth of the holes 225 so that an upper surface of each of theplugs 230 is generally coplanar with an upper surface of the first pieceof aluminum alloy 220 when the plugs are placed in the holes. Suitably,each of the plugs 230 has a diameter that is slightly less than thediameter of each of the holes 225 to facilitate insertion of the plugsinto the holes. In one suitable embodiment, each of the plugs 230 has adiameter that is between about 100 micrometers and about 400 micrometerssmaller than the diameter of the holes 225, more suitably about 200micrometers to about 300 micrometers smaller, and more suitably about250 micrometers smaller than the diameter of the holes 225. It isunderstood, however, that the plugs 230 and holes 225 can have differentrelative diameters without departing from the scope of this invention.

The tool 50 and pin 52 are used to spot friction stir weld together theplugs 230, the first piece of magnesium alloy 210, and the first pieceof aluminum alloy 220. The tool 50 and pin 52 can be rotated eithercounterclockwise, as indicated by arrow 54 in FIG. 9D, or clockwise. Thetool 50 and pin 52 are aligned with the center of one of the plugs 230and lowered so that the pin extends into the magnesium plug 230 and spotweld the first piece of aluminum alloy 220 to the first piece ofmagnesium alloy. This process is repeated for all of the plugs 230 toform spot welds 264 between the first piece of magnesium alloy 210 andthe first piece of aluminum alloy 220 are formed at each location asillustrated in FIG. 10.

In another suitable embodiment, each of the plugs 230 has a diameterthat is 50 to 100 percent larger than the diameter of the pin 52. Thus,if the pin 52 has a diameter of about 4 mm then each of the plugs wouldsuitably have a diameter between about 6 mm and about 8 mm. It isunderstood that the pin 52 and plugs 230 can have other diameterswithout departing from the scope of this invention.

A plurality of clamps 60 can be used to secure the pieces 210, 220 inplace during the welding process. It is contemplated that other meansfor securing the pieces 210, 220 during the welding process can be usedbesides the illustrated clamps 60 and that more or fewer clamps can beused.

As illustrated in FIGS. 10 and 11, a workpiece assembly, indicatedgenerally at 262, comprising the first piece of magnesium alloy 210 andthe first piece of aluminum alloy 220 being welded together along theweld seam 264. The weld seam 264 is discontinuous and defined bydiscrete spots of a stirred mixture of magnesium and aluminum alloys.

Experiment

In one experiment, pieces of an aluminum alloy (6061 Al) were welded topieces of a magnesium alloy (AZ31B Mg) using different weldingtechniques including conventional butt welding, conventional single-passlap welding, conventional dual-pass lap welding, the modifiedsingle-pass friction stir welding process described above andillustrated in FIGS. 3 and 4, and the modified double-pass friction stirwelding process described above and illustrated in FIGS. 6A-8. Tworeferences (or standards) were also formed using the same weldingtechniques used to weld together the pieces of 6061 Al and AZ31B Mg. Oneof the references was formed by welding a piece of AZ31B Mg to anotherpiece of AZ31B Mg and the other reference was formed by welding a pieceof 6061 Al to another piece of 6061 Al. Each of the pieces used in theexperiment was cut from 1.6 millimeter thick sheets of either AZ31 Mgalloy or 6061-T6 Al alloy and cleaned with a stainless steel brush toremove any surface oxides or other debris therefrom. The nominalchemical compositions of 6061 Al and AZ31B Mg are provided below inTable 1.

TABLE 1 Composition of workpiece materials (percent by weight) Si Cu MnMg Cr Zn Ti Fe Al 6061 Al 0.62 0.28 0.08 0.89 0.19 0.02 0.01 0.52 97.39AZ31B — — 0.5 95.5 — 1.0 — — 3.0 Mg

A 2.2 kW (3 HP) Lagun FTV-1 milling machine equipped with a H13 steeltool was used to friction stir weld the pieces of material together. Thetool had a concave shoulder with a 10 mm diameter. A 4 mm diameterthreaded pin depended from the tool. For butt welding, the pin lengthwas 1.3 mm and, for lap welding (both conventional and modified) the pinlength was 1.5 mm. Additional conventional lap welding was alsoconducted with using a pin having a length of 2.3 mm.

During the friction stir welding process, the tool was rotatedcounterclockwise, and tilted forward (i.e., in the direction of movementof the tool) 3 degrees. Each of the pieces was clamped down tight usingtwo, opposing steel fingers located about 10 mm away from the weld line.The tool and pin were cleaned after each welding pass by plunging itinto a fresh piece of 6061 Al. This removed any material potentiallystuck on the tool and/or pin from the previous weld. Two differentrotation speeds 1,400 rpm and 800 rpm were initially used. Besides oneexception, the joint strength was significantly lower when the rotationspeed was set at 800 rpm. As a result, the rotation speed was fixed at1,400 rpm after several welds were made at 800 rpm. The travel speed(i.e., the speed at which the tool and thereby the pin is moved alongthe weld line) was set at 38 mm/minute

Conventional Butt Welding

Eleven conventional butt welds were made during this experiment and theweld conditions and results are listed in Table 2. As seen in Table 2,the conventional butt welds formed between two pieces of 6061 Al or twopieces of AZ31 Mg were relatively strong (e.g., over 2,000 Newtons). Theother nine conventional butt welds were made to weld AZ31 Mg to 6061 Al.AZ31 Mg was either on the advancing or retreating side of the tool. Thetool axis was positioned along the joint (no offset) or shifted 1.5 mm(1.5 mm offset) toward either 6061 Al or AZ31 Mg. Three of the ninewelds were relatively weak, three were relatively strong and the otherthree were moderate. Thus, the strength of butt welds joining the piecesof 6061 Al and AZ31 Mg was inconsistent and therefore unpredictable.

TABLE 2 Conventional Butt welds Rotation Travel Pin Tool TensileStandard Speed Speed Length offset Load Deviation # Joint (rpm) (mm/min)(mm) (mm) (N) (+/−N) B-1 Al to Al 1400 38 1.3 0 3109 19 B-2 Mg to Mg1400 38 1.3 0 2580 102 B-3 Mg (ret) to Al 1400 38 1.3 0 1318 249 (adv)B-4 Al (ret) to Mg 800 38 1.3 0 1337 247 (adv) B-5 Al (ret) to Mg 140038 1.3 0 2055 274 (adv) B-6 Al (ret) to Mg 800 38 1.3 1.5 into 2590 73(adv) Mg B-7 Al (ret) to Mg 1400 38 1.3 1.5 into 2109 217 (adv) Mg B-8Al (adv) to Mg 1400 38 1.3 1.5 into <400 — (ret) Mg B-9 Al (ret) to Mg800 38 1.3 1.5 into Al <400 — (adv) B-10 Al (ret) to Mg 1400 38 1.3 1.5into Al 1589 — (adv) B-11 Mg (ret) to Al 1400 38 1.3 1.5 into Al <400 —(adv)

Conventional Single-Pass Lap Welding

Six conventional single-pass lap welds were made during this experimentand the conditions and results are listed in Table 3. Each of the lapwelds were was positioned along a centerline of a 38 mm overlap. Thatis, the overlap between the welded pieces was 38 mm and the weld wasmade along the centerline of the overlap (i.e., 19 mm from either edgeof the overlap).

As seen in Table 3, the conventional lap welds formed between two piecesof 6061 Al or two pieces of AZ31 Mg were relatively strong (e.g., over2,000 Newtons). The other four conventional lap welds were used to weldAZ31 Mg to 6061 Al. Three of the four welds were relatively weak whileone was moderate. Thus, the overall strength of butt welds joining thepieces of 6061 Al and AZ31 Mg was relatively low, inconsistent, andunpredictable.

TABLE 3 Conventional lap welds (single pass) Rotation Travel Pin TensileStandard Speed Speed Length Load Deviation # Joint (rpm) (mm/min) (mm)(N) (+/−N) CL-5 Al to Al 1400 38 1.5 3356 54 CL-6 Mg to Mg 1400 38 1.52463 190 CL-1 Al (top) to Mg (bottom) 1400 38 1.5 862 25 CL-2 Mg (top)to Al (bottom) 1400 38 1.5 1077 6 CL-3 Al (top) to Mg (bottom) 1400 382.3 554 5 CL-4 Mg (top) to Al (bottom) 1400 38 2.3 978 90

Conventional Dual-Pass Lap Welding

One conventional dual-pass lap welding was formed to determine how muchthe joint strength could be increased by making a second pass. Thewelding conditions of the dual-pass lap welding are listed in Table 4.The strength of the weld was more than double that of the single passweld (see CL-2 and CL-4).

TABLE 4 Conventional lap welds (dual pass) Rotation Travel Pin TensileStandard Speed Speed Length Load Deviation # Joint (rpm) (mm/min) (mm)(N) (+/−N) CL-7 Top: Mg 1400 38 1.5 2269 31 and 1st pass; Bottom: Al and2nd pass

Modified Single-Pass Lap Welding

The welding conditions of the single-pass modified friction stir weldingare provided in Table 5. A small piece of the bottom-sheet material, 76mm long, 19 mm wide, and 1.6 mm thick, was butt welded to the top sheetwith pin penetration into the bottom sheet. The 19 mm width of the smallpiece was mainly for the space required for clamping. If the clampspermitted, the width of the small piece could have been less. When AZ31Mg was on the top, whether it was the top sheet or the small piece, wasplaced on the advancing side of the tool. This was because, as will beshown subsequently, butt welds were significantly weaker with 6061 Al onthe advancing side.

TABLE 5 Modified lap welds (single-pass) Tool Rotation Travel Pin offsetTensile Standard Speed Speed Length at top Load Deviation # Joint (rpm)(mm/min) (mm) (mm) (N) (+/−N) ML-5 Top: Al (ret) 800 38 1.5 1.5 1808 8and small Mg into (adv); small Bottom: Mg Mg ML-1 Top: Al (ret) 1400 381.5 1.5 2711 235 and small Mg into (adv); small Bottom: Mg Mg ML-2 Top:Mg (adv) 1400 38 1.5 1.5 1434 14 and small Al into (ret); Bottom: smallAl Al ML-3 Top: Mg (adv) 1400 38 1.5 0 993 98 and small Al (ret);Bottom: Al ML-4 Top: Al (ret) 1400 38 1.5 0 1797 136 and small Mg (adv);Bottom: Mg

Modified Dual-Pass Lap Welding

Modified dual-pass friction stir welding was similar in materialpositions except a second pass was made from the opposite side, with itscenterline 10 mm away from that of the first pass. The weldingconditions of the modified dual-pass lap welds are listed in Table 6.

TABLE 6 Modified lap welds (dual pass) Rotation Travel Pin Tool TensileStandard Speed Speed Length offset Load Deviation # Joint (rpm) (mm/min)(mm) (mm) (N) (+/−N) ML-6 Top: Al (ret) 1400 38 1.5 1.5 into 4530 87 andsmall Mg small (adv); Mg and Bottom: Mg small (adv) and small Al Al(ret) ML-7 Top: Al (ret) 1400 38 1.5 0 3559 116 and small Mg (adv);Bottom: Mg (adv) and small Al (ret)

Tensile Testing

The joint strength, which is provided in the above tables, wasdetermined by tensile testing normal to the weld. Welded coupons werecut in the direction normal to the weld into to 12 mm-wide tensilespecimens. The edges of the tensile specimens were polished smooth with320-grit grinding paper. For lap welds, a 1.6 mm-thick sheet was placedat each end of the tensile specimen to initially align the specimen withthe loading direction. A Sintech tensile testing machine was used, andthe speed of the crosshead movement was 1 mm/min. Two to four specimensfrom welds made under the same condition were tested.

Temperature Measurements

A computer-based data acquisition system was used along with K-typethermocouples for temperature measurements at 100 Hz during the frictionstir welding process of each of the samples. The thermocouple, with astainless steel sheath of 0.5 mm outer diameter, was placed in a 0.5mm×0.5 mm groove at the workpiece surface that ended 3 mm away from thepath of the tool axis. In both conventional and modified lap frictionstir welding the grooves were at the top surface of the lower sheet. Inbutt friction stir welding, on the other hand, they were at the bottomsurface of the workpiece.

Weld Microstructure

Transverse weld cross-sections were prepared by polishing and etching inthree steps. The first step was to etch the samples with a solutionconsisting of 10 ml acetic acid, 10 ml distilled water and 6-gram picricacid in 100 ml ethanol for 10 s (to reveal the AZ31 part of themicrostructure). The second step was to etch them with a solutionconsisting of 20-gram NaOH in 100 ml distilled water for 40 s (to revealthe grain structure in 6061 Al). The final step was to dip them in asolution consisting of 4-gram KMnO₄ and 2-gram NaOH in 100 ml distilledwater for 10 s (to make Al colorful). The 3-step etching procedureshowed Al, Mg, Al₃Mg₂ and Al₁₂Mg₁₇ all in different colors.

A JEOL JSM-6100 scanning electron microscope with energy dispersivespectroscopy (EDS) was used for chemical composition measurements. AHi-Star 2-D x-ray diffractometer with an area detector was used toidentify the intermetallic compounds.

Al—Mg Phase Diagram

A binary Al—Mg phase diagram is shown in FIG. 12 wherein there are twoeutectics. The first one is between the Al-rich phase (Al) and Al₃Mg₂,which is essentially Al₃Mg₂, and the second is between the Mg-rich phase(Mg) and Al₁₂Mg₁₇. Both eutectic temperatures, 450° C. for the formerand 437° C. for the latter, are far below the melting points of Al (660°C.) and Mg (650° C.). According to the Al—Mg phase diagram shown in FIG.12, when Al and Mg are heated up together such as during friction stirwelding, intermetallic compounds Al₃Mg₂ and Al₁₂Mg₁₇ can form, theformer on the Al side and the latter on the Mg side. Upon furtherheating, the eutectic reaction Mg+Al₁₂Mg₁₇ occurs at the eutectictemperature 437° C., which causes liquid to form. The eutectic reactionAl+Al₃Mg₂ occurs at the eutectic temperature 450° C. and also causesliquid to form. This liquid is referred to as constitutional liquation.At the room temperature, Al₃Mg₂ contains about 37 wt % Mg and Al₁₂Mg₁₇about 57 wt % Mg. The eutectic temperatures 437° C. and 450° C. are morethan 200° C. below the melting point of either Al or Mg, and they can bereached easily during Al-to-Mg friction stir welding to form liquidfilms along the interface between Al and Mg. Upon cooling, the twoeutectic reactions are reversed, and Al₃Mg₂ and Al₁₂Mg₁₇ are formed fromthe liquid.

Heat Input in Friction Stir Welding

Liquation in the weld during friction stir welding increases when theheat input is increased. The increase in liquation may result in moreliquid films forming along grain boundaries and, in the case of Al-to-Mgfriction stir welding, the Al/Mg interface. Since the liquid filmsweaken the Al/Mg interface under shearing force caused by the tool,cracking may occur along the interface.

FIG. 13 shows two hypotheses made based on two facts regarding the heatinput observed in friction stir welding. With respect to fact 1, insimilar-metal butt friction stir welding more heating occurs on theadvancing side than the retreating side. Both computer simulations andtemperature measurements have shown higher peak temperatures on theadvancing side. As mentioned previously, the advancing side is the sidewhere material is pushed forward by the rotating tool, while theretreating side is the side where material is pushed backward. On theadvancing side, the tool rotates in the opposite direction of workpieceflow while on the retreating side it rotates in the same direction.Consequently, the material on the advancing side tends to experiencegreater shearing and heating than that on the retreating side.

For a lower conductivity material such as 304 stainless steel, thetemperature on the advancing side can be as much as 100° C. higher thanthat on the retreating side. For a higher conductivity material such asan Al or Mg alloy, the difference is often smaller. However, theliquation (eutectic) temperatures are rather low (437° C. and 450° C.).Furthermore, a relatively small temperature increase can significantlyincrease the fraction of liquid, that is, the extent of liquation. Forinstance, according to the Al—Mg phase diagram (FIG. 12) a material with60 wt % Mg and 40 wt % Al has a melting temperature range of only about10° C. Thus, this material begins to liquate at the eutectic temperature437° C. and melts completely at about 447° C.

In similar-metal butt friction stir welding, more heating occurs in 6xxxAl alloys than in AZ (Mg—Al—Zn) or AM (Mg—Al—Mn) Mg alloys. Insimilar-metal butt friction stir welding, higher peak temperatures (100°C. higher on the advancing side and 80° C. on the retreating side, bothat 10 mm from the joint line) have been observed in 6040 Al than in AZ31Mg. Similar trends have been observed in studies on similar-metalfriction stir spot welding where heat is generated by a rotating butstationary tool. The stir zone at the tool shoulder has been observed tohave a higher peak temperature (about 80° C. higher) in 6111 Al than inAZ91 Mg. A higher torque and heat input (almost three times higher heatinput) have been observed in 6061 Al than in AM60 Mg. A higher torqueand heat input (twice higher heat input) have been observed in 6061 Althan in AZ91 Mg.

With respect to fact 2, it has been observed in similar-metal frictionstir spot welding that AZ and AM Mg can liquate much more easily than6xxx Al. In AZ and AM Mg (and most other Mg alloys because Al is themost widely used alloying element in Mg alloys) Al₁₂Mg₁₇ is present toreact with the surrounding Mg-rich matrix to form liquid at 437° C.(FIG. 12). The liquid films at interface between the tool and the stirzone can cause tool slippage, while those along grain boundaries withinthe stir zone can decrease its resistance to tool rotation.Consequently, the torque and the work it does, which contributes tonearly all of the heat input, are significantly lower in welding AZ andAM Mg than 6xxx Al. Second, with its face-centered cubic structure, Alhas more slip planes available for deformation than Mg, which ishexagonal close-packed (hcp) in structure. Thus, as compared to Mg, Alis more deformable. It has been noted in similar-metal butt frictionstir welding that the stir zone was twice bigger in cross-section in6040 Al than in AZ31 Mg, which perhaps suggest more heating by viscousdissipation in the former.

Based on the two facts, two hypotheses can be made regardingdissimilar-metal friction stir welding of 6xxx Al to AZ or AM Mg withthe same tool at the same rotation speed and travel speed. Hypothesis 1is that a higher heat input can be expected in butt friction stirwelding with Al on the advancing side. Hypothesis 2 is that a higherheat input can be expected with a larger Al/tool contact area. A largerAl/tool contact area can exist in the following two cases: first, withtool offset to Al in butt friction stir welding and, second, with Al onthe top in lap friction stir welding. Regarding the first case, thedifference can be expected to be more significant with Al on theadvancing side in view of Hypothesis 1.

These hypotheses will be used below to explain the effect of materialpositions on the heat input in Al-to-Mg friction stir welding of thisexperiment.

Butt Welding

The effect of material positions on the joint strength in butt frictionstir welding is shown in FIG. 14. First, material positions have asignificant effect on the joint strength. The difference can be as highas a factor of about two to three. Second, the joint strength is higherwith AZ31 Mg on the advancing side. Third, increasing tool offset toAZ31 Mg improves the joint strength.

Based on the two hypotheses mentioned previously, with the same tool atthe same rotation speed and travel speed, the effect of materialpositions on the heat input in Al-to-Mg butt friction stir welding canbe predicted as shown by the arrow indicating the direction ofdecreasing heat input in FIG. 14. First, the heat input can be higher inbutt friction stir welding with Al on the advancing side (welds B-11 andB-3) than with Al on the retreating side (welds B-5 and B-7). Second,with Al on the advancing side the heat input can be higher with tooloffset to Al (weld B-11) than without any offset (weld B-3). Third, theheat input can be lower with tool offset to Mg (weld B-7) than withoutany offset (weld B-5), but the difference is likely to be smallerbecause Al is on the retreating side.

As shown in FIG. 14, the measured peak temperatures are in agreementwith the prediction. The thermocouples were 3 mm away from the path ofthe tool axis and 0.25 mm above the bottom surface of the workpiece. Asmentioned previously, in butt friction stir welding of 6040 Al to AZ31Mg without tool offset, higher peak temperatures (50° C. higher on theadvancing side and 30° C. on the retreating side) were observed with6040 Al on the advancing side, which are consistent with welds B-3 andB-5 in FIG. 14.

FIGS. 15A and 15B compares the thermal cycles measured in welds B-7 andB-11. In weld B-11, where Al is on the advancing side, the peaktemperature is 376° C. on the advancing side and 364° C. on theretreating side, the average being 370° C. In weld B-7, where Mg is onthe advancing side, the peak temperature is 286° C. on the advancingside and 306° C. on the retreating side, the average being 296° C.,which is 74° C. lower than the average peak temperature of 370° C. inweld B-11. In similar-metal butt friction stir welding, as mentionedpreviously, the peak temperature is higher on the advancing side.However, weld B-7 (and weld B-5 as well) shows that this can be reversedin dissimilar-metal butt friction stir welding.

As shown in FIG. 14, weld B-11 is significantly weaker than weld B-3,weld B-3 is significantly weaker than weld B-5, and weld B-5 is similarto weld B-7 in strength. A similar pattern exists in the measuredaverage temperatures. Thus, a close correlation seems to exist betweenincreasing heat input and decreasing joint strength. Liquation increaseswith increasing heat input or temperature. With more liquation, moreliquid can form along the Al/Mg interface to promote cracking under theshearing action of the tool and form brittle intermetallics both alongthe interface and grain boundaries inside the stir zone upon cooling.The joint strength can be significantly reduced.

Although FIG. 14 can explain how material positions can affect the jointstrength through the heat input and hence liquation, other factors mayalso affect the joint strength. For instance, interlocking between Mgand Al can improve the joint strength, so can similar-metal bonding(such as Al-to-Al and Mg-to-Mg, as will be shown subsequently inmodified lap welding). On the other hand, excessive mixing between Aland Mg can provide more interface area for Al to react with Mg to causeliquation and decrease the joint strength.

FIGS. 16A and 16B are transverse cross-sections of welds B-7 and weldB-11, respectively. The Al/tool contact area in weld B-7 is the same asthe Mg/tool contact area in weld B-11. In weld B-7 Al penetrates deepinto the stir zone, which can promote interlocking and improve the jointstrength. However, there is no Mg penetration into the stir zone in weldB-11. In fact, a long open crack exists along the interface between Mgand the stir zone over half the thickness of the workpiece. There mightbe two reasons for the differences. First, with its good deformabilityAl can move to the back of the rotating tool from the retreating sideeven though shearing is less there than the advancing side. Computersimulation has shown that material particles at the advancing side canenter into the retreating side but not the other way around. With itslower deformability, however, Mg is less able to move far away from theretreating side. Second, the higher heat input and hence liquation inweld B-11 could have caused a continuous liquid film to exist along theinterface between Mg and the stir zone over half the thickness of theworkpiece. The slippage caused by the liquid film could have kept Mgfrom being dragged deep into the stir zone. The large open crack and thecontinuous intermetallic layer along the interface both suggestliquation there. The crack caused weld B-11 to break even before tensiletesting. Thus, lack of interlocking caused by unfavorable material flowand more liquation caused by the higher heat input could have bothcontributed to the low joint strength of weld B-11. The microstructureof weld B-3 (not shown) indicated heavy liquation within the stir zonedue to relatively high heating and excessive mixing between Al and Mgcaused by zero offset (equal volume of Al and Mg exposed to the pin).

Single-Pass Conventional Lap Welding

FIG. 17 shows a single-pass conventional lap weld with AZ31 Mg on thetop (CL-2). The effect of material positions on the joint strength insingle-pass conventional lap friction stir welding is shown in FIG. 18.First, material positions have a significant effect on the jointstrength. The difference can be a factor of two. Second, the strength ishigher with AZ31 Mg on the top. Third, the strength is higher with the1.5 mm pin length than 2.3 mm. Fourth, for dissimilar-metal frictionstir welding between AZ31 Mg and 6061 Al, the highest strength in aconventional lap weld (CL-2) is much lower than that in a butt weld (B-7in FIG. 14), about one half. Butt welds are stronger mainly because lapwelds are subjected to shearing/peeling forces during tensile testingwhile butt welds are not.

The effect of material positions on the heat input in conventional lapfriction stir welding of 6xxx Al to AZ or AM Mg is predicted in FIG. 18.According to Hypothesis 2, with the same tool at the same rotation speedand travel speed, a higher heat input can be expected with a largerAl/tool contact area, that is, with 6xxx Al on the top to increase theAl/tool contact area. Thus, a higher heat input can be expected in weldsCL-3 and CL-1 than in welds CL-4 and CL-2. With a longer pin penetratinginto the lower sheet, a higher heat input can be expected in weld CL-3than weld CL-1 and in weld CL-4 than weld CL-2.

To verify that the heat input is higher with Al on the top and with alonger pin, temperature measurements were conducted. The thermocoupleswere located 3 mm away from the path of the tool axis and 0.25 mm belowthe top surface of the lower sheet. As shown in FIG. 18, the peaktemperature is 77° C. higher with 6061 Al on the top (weld CL-1) than atthe bottom (weld CL-2). Thus, this confirms the higher heat input inAl-to-Mg lap friction stir welding with Al on the top. Furthermore, thepeak temperatures are higher with a longer pin, that is, 52° C. higherin weld CL-3 than CL-1 and 40° C. higher in weld CL-4 than CL-2.

FIGS. 19A and 19B show transverse cross-sections of welds CL-1 and CL-2,respectively. In weld CL-1 (FIG. 19A) thick intermetallic compounds anda crack are present along the interface between the Al stir zone and theAZ31 Mg at the bottom. The brittle intermetallics and the crack musthave contributed to the low joint strength of the weld. As mentionedpreviously, it has been observed in lap friction stir welding a verythick layer of intermetallics at the interface between Al-7.5Si (top)and AZ31 Mg (bottom) even though the pin never touched AZ31 Mg. Thus,slight or no pin penetration into AZ31 Mg does not really matter much.Instead, putting AZ31 Mg on the top might work better (as shown by weldCL-2).

EDX (energy-dispersive x-ray) analysis showed the lighter layer next to6061 Al (inset on right) contained about 39 wt % Mg, which is close tothe 37 wt % Mg for Al₃Mg₂. The darker layer next to AZ31 Mg containedabout 63 wt % Mg, which is reasonably close to the 57 wt % Mg forAl₁₂Mg₁₇. EPMA (electron probe microanalysis) confirmed thecompositions. X-ray diffraction (XRD) also confirmed the presence ofAl₁₂Mg₁₇ and Al₃Mg₂.

The intermetallic layers in weld CL-1 (FIG. 19A) suggests that heatingduring friction stir welding was high enough to cause Al and Mg to reactwith each other and form liquid along the interface, that is,constitutional liquation. The Mg near the Al stir zone does not appearto be stirred (no flow lines visible in AZ31 Mg in inset on right),possibly because of the lower deformability of Mg or tool slippage byliquid films formed by liquation or both. Upon cooling, Al₁₂Mg₁₇ andAl₃Mg₂ formed from the liquid by eutectic reactions (FIG. 12).

EDX showed the particle inside the crack at the interface (inset on leftin FIG. 9A) contained about 60 wt % Mg, close to the 57 wt % Mg ofAl₁₂Mg₁₇. This suggests that liquation occurred here and the liquid filmcaused the stir zone to be separated from AZ31 Mg under shearing by therotating tool. (It is worth mentioning that in friction stir weldingcavities can form in the stir zone by material flow without liquation.)With a longer pin (2.3 mm instead of 1.5 mm) to penetrate deeper intoAZ31 Mg, that is, in weld CL-3, much more intermetallics formed at theinterface near the pin tip due to more heating (52° C. higher peaktemperature as shown in FIG. 18).

In weld CL-2 (FIG. 19B), the intermetallics are thinner and crackssmaller and shorter along the interface between the Mg stir zone and the6061 Al at the bottom. The region of 6061 Al next to the Mg stir zoneappears to be well stirred (flow lines visible in inset on left). Allthese suggest that, as compared to weld CL-1, liquation wassignificantly less, consistent with the lower heat input in weld CL-2(77° C. lower peak temperature as shown in FIG. 18). With a longer pinto penetrate deeper into 6061 Al than in weld CL-2, that is, in weldCL-4, more cracks and intermetallics formed at the interface near thepin tip due to more heating (40° C. higher peak temperature as shown inFIG. 18).

Single-Pass Modified Lap Welding

In order to improve the strength of Al-to-Mg lap welds, conventional lapfriction stir welding was modified. FIGS. 20A and 20B are provided forthe comparison of conventional lap welding with the modified lapfriction stir welding, as described therein, of dissimilar metals A andB. With conventional lap welding (FIG. 20A), metal A is placed on top ofmetal B. As mentioned previously, with only slight or even no pinpenetration into metal B, metals A and B can still react with each otherand form a rather thick layer of intermetallics at the interface. Withmodified lap welding (FIG. 20B), metal A is still placed on top of metalB but with a small piece of metal B next to it. In one example, metal Acan be 6061 Al and metal B can be AZ31 Mg or vice versa.

FIG. 21 shows a single-pass modified lap weld with AZ31 Mg and a smallpiece of 6061 Al at the top (ML-2). As mentioned previously (FIG. 20B),modified lap welding involves both butt and lap welding. In light of thebutt welding result (FIG. 14), all welds were made with AZ31 Mg on theadvancing side, either as the top sheet or the small piece at the top.

The effect of material positions on the joint strength in single-passmodified lap friction stir welding is shown in FIG. 22. First, materialpositions have a significant effect on the joint strength, and thedifference can be a factor of two to three. Second, the strength ishighest in the weld (ML-1) with a tool offset to the small piece of AZ31Mg. This is consistent with the butt welding result (FIG. 14). This alsoallows much Mg-to-Mg lap welding, which is much stronger than Al-to-Mglap welding because of the absence of cracks and intermetallics. Third,weld ML-1 (2,711N) matched in strength the similar-metal lap weld CL-6(2,463N as shown in Table 2) between AZ31 Mg and itself.

FIG. 23 compares the tensile test curves of the best single-passconventional lap weld CL-2 and the best single-pass modified lap weldML-1. Weld ML-1 failed at a significantly higher strength and elongationthan weld CL-2.

The effect of material positions on the heat input in modified lapfriction stir welding is predicted in FIG. 22. According to Hypothesis2, with the same tool at the same rotation speed and travel speed, ahigher heat input can be expected with a larger Al/tool contact area.Since the contact area between Al and the tool (shoulder and pin)decreases in the order of ML-2, ML-3, ML-4 and ML-1, the heat input canbe expected to decrease in the same order. This prediction is confirmedby the peak temperatures measured during friction stir welding. Thethermocouples were on the advancing side and located 3 mm away from thepath of the tool axis and 0.25 mm below the top surface of the lowersheet. Going from weld ML-3 to weld ML-4, the bottom sheet changes from6061 Al to AZ31 Mg, which is lower in thermal conductivity (167 vs. 96W/m° C.). The fact that the peak temperature still decreases suggeststhe effect of thermal conductivity difference is not very significant.

FIGS. 24A and 24B show transverse cross-sections of welds ML-3 and ML-1,respectively. In weld ML-3 (FIG. 24A), Al and Mg interpenetrates deepinto each other, and this can be expected to promote interlocking andimprove the joint strength. Unfortunately, the heat input was relativelyhigh (FIG. 22) and it caused much liquation and a long crack along mostof the Mg—Al interface (see insets). Under the shearing/peeling actioninherent during tensile testing of lap welds, the crack can open upeasily and lead to premature failure. In weld ML-1 (FIG. 24B), however,there was significantly less heating (FIG. 22) to cause liquation.Furthermore, strong Mg-to-Mg bonding exists at the interface between thestir zone and the bottom sheet without cracks or intermetallics. By theway, the light gray straight lines in AZ31 Mg are twin lines instead ofscratches left on the sample due to poor polishing.

As shown in FIG. 22, however, the joint strength increases in the orderof ML-3, ML-2, ML-4 and ML-1. That is, weld ML-2 is stronger than weldML-3 in spite of the higher heat input in the former. As compared toweld ML-3, weld ML-2 allows more of stronger Al-to-Al lap welding andless of weaker Al-to-Mg lap welding. This can explain why weld ML-2 isstronger than weld ML-3.

A weld such as ML-1 can be prepared as follows. 6061 Al sheets, AZ31 Mgsheets, and small AZ31 Mg sheets can be sheared with parallel edges tothe predetermined width. With 6061 Al on top of AZ31 Mg and positioned,both can be clamped down simultaneously from one side. After putting thesmall AZ31 Mg next to 6061 Al and clamping down from the opposite side,the lateral position of the joint line relative to the pin can be fineadjusted just like in butt welding. Since the small AZ31 Mg is free tomove, its close fit-up with 6061 Al is guaranteed regardless how precisethe dimensions of the sheets are. The small AZ31 Mg can then be buttwelded to 6061 Al with pin penetration into the backing plate. This, infact, can be easier to do than ordinary butt friction stir weldingbecause pin penetration into the backing plate (i.e., support surface)does not have to be avoided.

Dual-Pass Lap Welding

FIG. 25 shows the effect of material positions on the strength ofdual-pass lap welds made between AZ31 Mg and 6061 Al. For modified lapwelds, AZ31 Mg was on the advancing side in each pass. Weld ML-6 isstronger than weld ML-7. The first pass (top) in weld ML-6 is equivalentto the single-pass weld ML-1 (FIG. 22), and that in weld ML-7 to thesingle-pass weld ML-4. Since weld ML-1 is stronger than weld ML-4, thefirst pass in the dual-pass weld can be expected to be stronger in weldML-6 than in weld ML-7. The second pass (bottom) in weld ML-6 isequivalent to the single-pass weld ML-2 (FIG. 22), and that in weld ML-7to the single-pass weld ML-3. Since weld ML-2 is stronger than weldML-3, the second pass in the dual-pass weld can also be expected to bestronger in weld ML-6 than in weld ML-7.

Weld ML-6 is stronger than the dual-pass conventional lap weld CL-7 by afactor of about two (FIG. 26). This significant difference is consistentwith the results shown previously in FIG. 23, where the single-passmodified weld ML-1 is also about twice stronger than the single-passconventional lap weld CL-2. The tensile test curves of welds CL-7 andML-6 are shown in FIG. 26. Weld ML-6 fails at a much higher strain aswell as load. Weld CL-7 failed through the weld as all other cases, butweld ML-6 failed in the 6061 Al base metal. This is the advantage ofdual-pass modified lap welds since failure in the base metal is anassurance of strong bonding.

Modified Friction Stir Spot Welding

FIG. 27 graphically illustrates tensile test strength results frommodified friction stir spot welding of 1.6 mm thick 6061 Al to 1.6 mmthick AZ31 Mg. The welding was done with a tool having 1 cm shoulderdiameter, 4 mm pin diameter, and 1.3 mm pin length at 1400 rpm rotationspeed with 5 second dwell time after reaching the desired penetration.Specifically, a hole was drilled (or punched) in the upper sheet, theupper sheet was then put on top of the lower sheet, and a plug of thelower sheet material is inserted into the hole in the upper sheet. Therotating pin was plunged into the plug to stir and cause bonding. Deeppin penetration into the lower sheet is not required and in fact, notdesired. It is believed that better joint strength is achieved with thepin tip at or even slightly above the joint line (i.e., the originalinterface between the plug and the lower sheet). With reference to FIG.27, the stars denote the upper sheet is AZ31 Mg and its holes are filledwith 6061 Al plugs. Contrarily, the circles denote the upper sheet is6061 Al and its holes are filled with AZ31 Mg plugs.

A zero disc diameter, also indicated by “no plug” on FIG. 27, meansconventional spot welding, that is, no holes or plugs was used to weldthe Al and Mg sheets together. During convention friction stir spotwelding, the rotating pin is plunged into the upper sheet until theshoulder rubs on the upper sheet to generate sufficient friction heatand plasticize the workpiece material. The pin tip penetrates into thelower sheet to cause stirring and bonding between the upper and lowersheets. A keyhole (or crater) is left after the tool is withdrawn. Whenjoining dissimilar metals by conventional friction strip sport welding,pin penetration into the lower sheet should be minimized in order tominimize the reaction between metal to form liquid, which causescracking and the formation of brittle intermetallic compounds. So, thepin length should be such that the pin tip penetrates the lower sheetslightly when the shoulder rubs on the upper sheet.

How the material positions affect the joint strength of the resultantweld depends significantly on how they affect the heat input andmaterial flow during friction stir welding, both of which affect theformation of defects and hence the joint strength. At lower travelspeeds and higher rotation speeds, more heat is generated to causeliquation and hence cracking and intermetallic compounds to weaken theresultant weld. So, the heat input is likely to play a bigger role thanmaterials flow. At higher travel speeds and lower rotation speeds, onthe other hand, less heat is generated to cause liquation. However, thematerials may not be warm enough for sufficient plastic flow to keepchannels from forming and weakening the resultant weld. So, materialflow is likely play a bigger role than the heat input. In the presentstudy, the travel speed was set to 38 mm/min, which is low, and therotation speed was set to 1,400 rpm, which is intermediate. The resultsindicate that the heat input plays a bigger role than material flow inmost cases.

Within the range of experimental conditions in the present study, thefollowing conclusions, which can be useful for structure design infriction stir welding of dissimilar metals can be drawn:

-   -   Conventional lap welding of metal A at top to metal B at bottom        can be modified to improve the joint strength by butt welding a        small piece of metal B to metal A with pin penetration into the        metal B at the bottom (which can be easier to do than ordinary        butt welding because pin penetration into the backing plate is        not a problem here). The highest joint strength in Al-to-Mg        modified lap friction stir welding can double that in        conventional lap friction stir welding and match that in        Mg-to-Mg lap friction stir welding. This is because        similar-metal bonding, which is stronger than dissimilar-metal        bonding, can exist over most or the interface between the stir        zone and the bottom piece in modified lap welding.    -   A significant effect of material positions on the joint strength        has been demonstrated in Al-to-Mg butt, conventional lap and        modified lap friction stir welding, affecting the joint strength        by a factor of two or more.    -   The effect of material positions on the heat input has been        predicted and confirmed with temperature measurements during        Al-to-Mg butt, lap and modified lap friction stir welding. This        helps better understand the effect of material positions on the        joint strength because the heat input affects the formation of        liquid and hence cracks and brittle intermetallic compounds.    -   If the heat input is higher in A-to-A friction stir welding than        in B-to-B friction stir welding under identical welding        conditions, the heat input in A-to-B friction stir welding can        be higher with A on the advancing side (in butt friction stir        welding) and with a larger A/tool contact area (that is, with        tool offset to A in butt friction stir welding or with A at the        top in lap friction stir welding).    -   A three-step color etching procedure has been developed to show        Mg, Al, Al₃Mg₂ and Al₁₂Mg₁₂ all in different colors, thus        enabling clear interpretation of the microstructural        constituents, material flow, mixing, and evidence of liquation.    -   Material positions that favor a lower heat input can be used to        increase the joint strength (as long as the heat input is not        too low, e.g., at high travel speeds or low rotation speeds, to        maintain sufficient plastic material flow to prevent channels        from forming and weakening the resultant weld).    -   In butt friction stir welding of 6xxx Al to AZ or AM Mg, the        following material positions favor a lower heat input: Mg on the        advancing side and Al on the retreating side, with tool offset        to Mg.    -   In conventional lap friction stir welding of 6xxx Al to AZ or AM        Mg, the following material positions favor a lower heat input:        Mg on the top and Al at the bottom, with slight (e.g., 0.1 mm)        pin penetration into Al.    -   In modified lap friction stir welding of 6xxx Al to AZ or AM Mg,        the following material positions favor a lower heat input: Mg at        the bottom, Al on the top on the retreating side, and a small        piece of Mg on the top on the advancing side, to which the tool        offsets.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. A workpiece assembly comprising: a first piece ofa first metal; a first piece of a second metal overlying at least aportion of the first piece of the first metal, the first piece of secondmetal having a plurality of hole locations therein with each of the holelocations including material from a plug formed from the first metal;and a plurality of weld spots securely joining the first piece of firstmetal and the first piece of second metal, each weld spot: beingcentered on one of the hole locations and separated from other ones ofthe weld spots by the first piece of second metal along a lineardirection, and comprising a stirred mixture of the first metal from theplug in the hole location and the second metal, the stirred mixtureextending through a cross-section of the entire weld spot along thelinear direction, each of the weld spots filling the hole locations inthe first piece of second metal and located over respective portions ofthe first piece of the first metal.
 2. The workpiece assembly of claim 1wherein the first metal is dissimilar from the second metal.
 3. Theworkpiece assembly of claim 1 wherein the first metal is a first metalalloy and the second metal is a second metal alloy that is dissimilarfrom the first metal alloy.
 4. The workpiece assembly of claim 2 whereinthe first metal is a magnesium alloy and the second metal is an aluminumalloy.
 5. The workpiece assembly of claim 4 wherein each weld spot isdefined by a stirred mixture of magnesium and aluminum alloys.
 6. Theworkpiece assembly of claim 1 wherein each of the hole locations isformed from a hole having a first diameter and a plug having a seconddiameter less than the first diameter of the hole.
 7. The workpieceassembly of claim 6, wherein the stirred mixture in each weld spot fillsthe holes.
 8. The workpiece assembly of claim 1, wherein each holelocation is spaced apart from the other hole locations along atransverse joint line extending in the linear direction, by an unweldedportion of the second metal.
 9. The workpiece assembly of claim 1,wherein at each weld spot, the stirred mixture extends from an uppersurface of the first piece of the second metal, through the first pieceof the second metal and into a portion of the first piece of the firstmetal.
 10. The workpiece assembly of claim 1, wherein the weld spotsexhibit friction-stir welding characteristics throughout a cross-sectionof the weld spots.
 11. An apparatus comprising: a first piece of a firsttype of metal; a first piece of a second type of metal having aplurality of holes therein and overlying at least a portion of the firstpiece of the first type of metal, the first type of metal beingdissimilar from the second type of metal and the holes being spacedapart from one another; and in each of the holes, a plug of the firsttype of metal being configured and arranged with the first piece of thefirst type of metal and the first piece of the second type of metal to,in response to a friction stir welding tool being inserted into thehole, provide a stirred spot weld in each hole, the stirred spot weldoverlying a portion of the first piece of the first type of metal andhaving a stirred mixture extending through a cross-section of the entirespot weld and including portions of the first piece of the first type ofmetal, the first piece of the second type of metal, and the plug, eachweld spot being centered on the center of one of the plugs and separatedfrom other ones of the weld spots along a linear direction by acontiguous portion of the first piece of the second type of metal. 12.The apparatus of claim 11, wherein each plug has a diameter of about 250microns less than a diameter of the hole in which the plug is in. 13.The workpiece assembly of claim 1, wherein the respective portions ofthe first piece of the first metal over which the weld spots are locatedare configured and arranged to support the plug in the respective holeswhile a friction stir welding tool is plunged into the plugs and createsthe stirred mixture.
 14. The apparatus of claim 11, wherein respectiveportions of the first piece of the first metal over which the weld spotsare respectively located are configured and arranged to support thestirred mixture in the respective holes while the friction stir weldingtool is inserted through the second piece of the second type of metaland partially into the first piece of the first type of metal.